1. Field of the Invention
The invention relates to thermoplastic polyurethane (TPU) composites. In one aspect the invention relates to TPU composites that are halogen-free and flame retardant while in another aspect, the invention relates to TPU composites that are halogen-free, flame retardant and comprise crosslinked, silane grafted ethylene vinyl acetate copolymer.
2. Description of the Related Art
Halogen-free, flame retardant (HFFR) thermoplastic polyurethane (TPU) compositions are replacing halogen-containing, flame retardant TPU compositions in a number of applications including, but not limited to, the insulation and protective jackets for the wire and cable associated with personal electronic devices. These HFFR TPU compositions can provide excellent flame retardant performance and mechanical properties including excellent flexibility. Moreover, these HFFR TPU compositions can fulfill the heat deformation testing (UL-1581) requirements at 150° C. which is particularly important for some applications and generally is not obtainable with a polymer matrix of uncrosslinked polyolefin. However, these HFFR TPU compositions are not without their limitations and in comparison with HFFR polyolefin compositions, often prove more costly and exhibit insulation resistance (IR) failure, poor smoke density and higher material density.
Ethylene vinyl acetate (EVA) copolymer has a good compatibility with TPU and also exhibits good flexibility. EVA with a low vinyl acetate content (30 weight percent (wt %) or less) displays higher electrical resistance and lower material density than TPU. EVA is also less expensive than TPU, and it does not contain a benzene structure in its molecular structure (aromatics structures contribute to the smoke density of a product). As such, EVA appears at first as an appropriate compound for blending with TPU so as to improve on the IR and smoke density properties of TPU without sacrifice of the mechanical property and flexibility of TPU. However, the heat deformation performance of TPU is destroyed upon its compounding with EVA and as such, HFFR TPU/EVA compositions cannot pass heat deformation specifications that require a deformation ratio lower than 50% at 150° C.